The present invention generally relates to systems for reproducing a video signal in a still picture reproduction, and more particularly to a still picture reproduction system for carrying out a still picture reproduction in a state where a magnetic tape is stopped at a position such that a noise does not appear in a reproduced picture, when carrying out the still picture reproduction by stopping the moving magnetic tape during a normal reproduction mode in which a pre-recorded video signal is reproduced from the moving magnetic tape.
Generally, when carrying out a still picture reproduction in a state where a video tape recorder is in a normal reproduction mode, for example, a moving magnetic tape is stopped. Video tracks formed obliquely to a longitudinal direction on the tape, having a video signal pre-recorded thereon, are repeatedly scanned by a plurality of rotary heads. During such a still picture reproduction mode, the inclination angle of scanning loci of the rotary heads with respect to the tape, is different from the inclination angle of the recorded video tracks. For this reason, a reproduced picture is obtained normally while a reproducing head scans over a recorded track which has been formed by a recording head having a gap of the same azimuth angle as the gap of the reproducing head. However, when the reproducing head deviates from this recorded track and scans over a recorded track which has been recorded by a recording head having a gap of an azimuth angle which is different from the azimuth angle of the gap of the reproducing head, the level of the reproduced output decreases and a noise is introduced in the reproduced picture.
In order to make sure that the noise appears at an inconspicuous position in the reproduced picture, the moving tape should be stopped at a predetermined position where the reproducing head scans over the track on the tape so that a point when the level of the reproduced video signal becomes a minimum is located within or in a vicinity of a vertical blankinq period of the video signal. Various systems have been previously proposed in order to stop the moving at the above predetermined position.
For example, a still picture reproduction system was proposed in a U.S. Pat. No. 4,246,616 in which the assignee is the same as the assignee of the present application.
In the conventional still picture reproduction system for reproducing the video signal, the video signal is recorded on video tracks on the tape. The video tracks extend obliquely to the longitudinal direction of the tape, with substantially no gap formed between the adjacent video tracks. A control signal is recorded on a control track which extends in the longitudinal direction of the tape, and the video tracks are formed by a plurality of rotary video heads having gaps of mutually different azimuth angles. The control signal is recorded on the control track interrelatedly with the formation of the video tracks by the video heads. A motor drives the tape to move or to stop. The plurality of video heads successively scan over the video tracks to pick up and reproduce the pre-recorded video signals. The gaps of the video heads have different height positions above the plane of rotation of the center of the tracks in the width direction of the track. The control signal is reproduced from the control track on the moving tape. A circuit delays the reproduced control signal by a predetermined delay time, to obtain a delayed signal, and this delayed signal is used to stop the motor and to stop the moving tape. The moving tape is stopped at a position relative to the video heads which perform the reproduction, so that the point when the level of the reproduced video signal becomes a minimum is within or in a vicinity of the vertical blanking period of the video signal. In the present specification, this position where the noise in the reproduced picture becomes a minimum, will simply be referred to as a most suitable position.
When carrying out the still picture reproduction, it is necessary to stop the moving tape at the most suitable position with respect to the video heads, by tape movement stopping means. Conventionally, as such tape movement stopping means, there was means for immediately stopping the rotation of a capstan motor by short-circuiting terminals of the capstan motor, means for stopping the moving tape by using the inertia of a capstan and a flywheel and by taking into account the magnitude of the braking with respect to the capstan motor, and means provided with a second control head for immediately stopping the rotation of the capstan motor when this second control head reproduces a control pulse.
However, in the above conventional tape movement stopping means, although the reproduced control signal is used as a reference signal for stopping the moving tape, there is no feedback to determine whether the noise is actually within the vertical blanking period and does not appear at a conspicuous position in the reproduced picture. Thus, after the control pulse is detected, a discrimination could not be performed to determine whether the moving tape had actually stopped at the most suitable position. Accordingly, in the conventional still picture reproduction system, when the various conditions with respect to the mechanisms in the video tape recorder changed as the video tape recorder was used for a long time, it became necessary to re-adjust the tracking every time such a change occurred. Such a change in the conditions included a stretch in a belt which transmits a rotational force to the capstan, a change in a frictional drag in a tape moving system, and the like, for example.
Another system was proposed in a U.S. Pat. Application S.N.389,241 filed June 17, 1982 and entitled "VIDEO SIGNAL SLOW-MOTION OR STILL PICTURE REPRODUCTION SYSTEM", now U.S. Pat. No. 4,531,162, in which the assignee is the same as the assignee of the present application. According to this proposed system, the level of the reproduced FM signal is detected while the tape moves at a speed which is slower than the tape moving speed at the time of the recording. The moving tape is stopped at a position where a minimum level of the reproduced FM signal occurs in a duration in which a head switching pulse is generated, which duration corresponds to the vertical blanking period. However, according to this proposed system, it was necessary to provide a detecting circuit which comprises a comparator or the like for detecting the minimum level of the reproduced FM signal, in addition to the video signal reproducing circuit. Further, the level of the reproduced FM signal changed depending on the state of contact between the tape and the head. For this reason, the minimum level of the reproduced FM signal could not always be detected accurately, and this proposed system suffered a disadvantage in that the moving tape could not be accurately stopped at the most suitable position when such a change occurred in the level of the reproduced FM signal.